In 47 CE, Scri­bo­nius Largus, court physi­cian to the Roman emper­or Claudius, described in his Com­po­si­tiones a method for treat­ing chron­ic migraines: place tor­pe­do fish on the scalps of patients to ease their pain with elec­tric shocks. Largus was on the right path; our brains are com­prised of elec­tri­cal sig­nals that influ­ence how brain cells com­mu­ni­cate with each oth­er and in turn affect cog­ni­tive process­es such as mem­o­ry, emo­tion and atten­tion.

The sci­ence of brain stim­u­la­tion – alter­ing elec­tri­cal sig­nals in the brain – has, need­less to say, changed in the past 2,000 years. Today we have a hand­ful of tran­scra­nial direct cur­rent stim­u­la­tion (tDCS) devices that deliv­er con­stant, low cur­rent to spe­cif­ic regions of the brain through elec­trodes on the scalp, for users rang­ing from online video-gamers to pro­fes­sion­al ath­letes and peo­ple with depres­sion. Yet cog­ni­tive neu­ro­sci­en­tists are still work­ing to under­stand just how much we can influ­ence brain sig­nals and improve cog­ni­tion with these tech­niques.

Brain stim­u­la­tion by tDCS is non-inva­sive and inex­pen­sive. Some sci­en­tists think it increas­es the like­li­hood that neu­rons will fire, alter­ing neur­al con­nec­tions and poten­tial­ly improv­ing the cog­ni­tive skills asso­ci­at­ed with spe­cif­ic brain regions. Neur­al net­works asso­ci­at­ed with atten­tion con­trol can be tar­get­ed to improve focus in peo­ple with atten­tion deficit-hyper­ac­tiv­i­ty dis­or­der (ADHD). Or peo­ple who have a hard time remem­ber­ing shop­ping lists and phone num­bers might like to tar­get brain areas asso­ci­at­ed with short-term (also known as work­ing) mem­o­ry in order to enhance this cog­ni­tive process. How­ev­er, the effects of tDCS are incon­clu­sive across a wide body of peer-reviewed stud­ies, par­tic­u­lar­ly after a sin­gle ses­sion. In fact, some experts ques­tion whether enough elec­tri­cal stim­u­la­tion from the tech­nique is pass­ing through the scalp into the brain to alter con­nec­tions between brain cells at all.

Notably, the neu­ro­sci­en­tist Györ­gy Buzsá­ki at New York Uni­ver­si­ty pre­sent­ed research con­duct­ed with cadav­ers, con­clud­ing that very lit­tle of the cur­rent admin­is­tered through tDCS actu­al­ly trav­els into the brain, per­haps under 10 per cent. Oth­er researchers report the oppo­site. Recent neu­roimag­ing stud­ies have shown sig­nif­i­cant increas­es in neu­ro­trans­mit­ter lev­els and blood­flow at the site of tDCS stim­u­la­tion dur­ing a sin­gle ses­sion. Still, in response to grow­ing con­cern, many researchers have begun to admin­is­ter tDCS over a peri­od of days for an addi­tive effect. Stud­ies have shown enhanced treat­ment effects (yet no increase in side effects) attrib­ut­able to repeat­ed ses­sions as opposed to a sin­gle ses­sion of tDCS.

Even more basic con­cerns about tDCS research need to be addressed; par­tic­u­lar­ly, tDCS pro­to­cols are rather incon­sis­tent between research labs. For exam­ple, one lab might admin­is­ter tDCS for 20 min­utes at the max­i­mum volt­age of 2 mA while anoth­er lab might admin­is­ter tDCS for 25 min­utes at 1 mA, and anoth­er still might admin­is­ter for 15 min­utes at 1.5 mA. Com­bin­ing each of these stud­ies into a lit­er­a­ture review proves time-con­sum­ing and con­fus­ing. We do not know yet what the opti­mal time and volt­age lev­els are for tDCS. Let’s say 1 mA is too low to ini­ti­ate neur­al changes and improve cog­ni­tive abil­i­ties. Then hand­fuls of papers and years of research could turn out to be quite unin­for­ma­tive.

Late­ly, the tech­nol­o­gy has been com­bined with cog­ni­tive train­ing to achieve long-term improve­ments. This is a nat­ur­al pro­gres­sion of the work. It is thought that tDCS allows neu­rons to fire more read­i­ly. Then on top of that, just like work­ing out a mus­cle, a cog­ni­tive train­ing task is an exer­cise that will work out the neu­rons in brain regions heav­i­ly involved with employ­ing that cog­ni­tive process (mus­cles). To take advan­tage of both of these tech­niques, shouldn’t we then encour­age those neu­rons and brain regions to work even hard­er dur­ing tDCS by engag­ing the spe­cif­ic brain areas being tar­get­ed with a cog­ni­tive task? In fact, stud­ies con­firm this the­o­ry, and show that height­ened per­for­mance and longer-last­ing improve­ments result from the com­bi­na­tion of cog­ni­tive train­ing with tDCS.

In a sev­er­al-year col­lab­o­ra­tion between the Cog­ni­tive Neu­roimag­ing Lab at the Uni­ver­si­ty of Michi­gan and the Work­ing Mem­o­ry and Plas­tic­i­ty Lab at the Uni­ver­si­ty of Cal­i­for­nia at Irvine, we have been inves­ti­gat­ing work­ing-mem­o­ry train­ing in con­junc­tion with tDCS. Dur­ing the train­ing task, par­tic­i­pants are asked to hold pro­gres­sive­ly more infor­ma­tion in their work­ing mem­o­ry while simul­ta­ne­ous­ly under­go­ing tDCS. Although the results are still lim­it­ed and some­what mixed, evi­dence sug­gests that the com­bi­na­tion of brain stim­u­la­tion and train­ing is more effec­tive in improv­ing work­ing mem­o­ry than either tech­nique alone. For the exper­i­men­tal tDCS group, bet­ter per­for­mance could be mea­sured even a year after our ses­sions, an improve­ment not found with place­bo con­trols. And our col­lab­o­ra­tion has even uncov­ered a nuance of com­bined work­ing-mem­o­ry train­ing and tDCS: par­tic­i­pants who began train­ing with a low­er base­line work­ing mem­o­ry improved more than those who began with a high­er base­line per­for­mance.

Clear­ly there is much more work to do to under­stand tDCS and cog­ni­tive train­ing. To cre­ate more con­sis­ten­cy in the lit­er­a­ture, researchers will need to inves­ti­gate opti­mal para­me­ters (such as time length and cur­rent inten­si­ty) for tDCS as a form of cog­ni­tive and ther­a­peu­tic enhance­ment. A next step is to under­stand the under­ly­ing neur­al mech­a­nisms of tDCS and cog­ni­tive train­ing, which will require a mul­ti­dis­ci­pli­nary approach using neu­roimag­ing tech­niques such as fMRI. This would then make it pos­si­ble for researchers to visu­alise (1) acti­va­tion of brain regions due to tDCS, (2) acti­va­tion due to tDCS and a cog­ni­tive task, and even (3) changes in acti­va­tion specif­i­cal­ly due to com­bined tDCS and cog­ni­tive train­ing over cog­ni­tive train­ing alone.

I am cau­tious­ly opti­mistic about the promise of tDCS; cog­ni­tive train­ing paired with tDCS specif­i­cal­ly could lead to improve­ments in atten­tion and mem­o­ry for peo­ple of all ages and make some huge changes in soci­ety. Maybe we could help to stave off cog­ni­tive decline in old­er adults or enhance cog­ni­tive skills, such as focus, in peo­ple such as air­line pilots or sol­diers, who need it the most. Still, I am hap­py to report that we have at least moved on from tor­pe­do fish.

– Tes­sa Abagis is a grad­u­ate stu­dent in cog­ni­tion and cog­ni­tive neu­ro­science at the Uni­ver­si­ty of Michi­gan. This arti­cle was orig­i­nal­ly pub­lished at Aeon and has been repub­lished under Cre­ative Com­mons.

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